I did some more testing and have a solution but no conclusive cause.
I compared the version on my laptop (HL2-Beta 4) and the current B6 version. The beta 4 version setup wasn't quite complete but the record and replay functioned as expected.
I then changed the settings on the Beta 4 install to match the B6 version and the same replay problem surfaced on the B4 version.
Ticking Setup|Audio|VAC1|Bypass VAC for recording playbackmade no difference on either build
When the problem occurs the record functions but playback will only work if setup/Audio/VAC1/ Direct I/Q is selected, then the audio recording will play but the Rx audio won't.
Reset of the database on the B4 version immediately corrected the problem.
I then spent some time comparing and selecting the various options but couldn't trigger the problem again on the B4 version.
My conclusion is that some combination of changes to the setup is triggering the problem but I wasn't ble to determine exactly which sequence of changes.
The only solution I found is to reset the database and reprogram the setup.